The most likely problem is that you were using low resolution settings for your images.
Did you use settings when taking photos to get as many images on the card as you could? If you did, you made a basic mistake: the more images you try to get on a card, the lower the overall quality will be. You ALWAYS want to take your photos at the highest quality settings possible. Your computer screen displays at about 72 dpi (dots per inch), that's the standard resolution for images that are ONLY going to be displayed on screen. You cannot enlarge a 72 dpi image to a reasonable print size and expect it to look good because all you are doing is making the dots (pixels) bigger. Memory cards are relatively cheap: buy a couple of 2 GB or 4 GB cards and you can get a lot of images on them without worrying about space.
When you say you put "tons of my photos and digital art on my sd card" it sounds like you did exactly that: made the files small enough to cram lots of them on the card. So the result is you have lots of low quality small files on the card.
When you are editing things in Photoshop, make sure you also have good resolution settings. If you are changing image sizes, make sure you preserve the resolution at about 300 dpi. And remember, again, that if you are starting with a low resolution image, you cannot make it into a high resolution image.
Standard rule: start out with high quality large files. You can ALWAYS remove data, but you CAN'T add in data that isn't there already.
Another standard rule: open your file. Immediately do a SAVE AS (call it whatever you want, maybe FileName_working or FileName_Rev1). That way you don't mess up your original, you are always working on a copy.
Another standard rule: do NOT edit JPEGs. Jpegs use what is called a "lossy" compression method. That means that everytime you open, edit and resave that file, it throws away some data. Eventually the image quality degrades. Again, when you do the SAVE AS, change the file type to Photoshop. (.psd). Work in the native Photoshop file type (which also gives you access to all the cool stuff Photoshop can do). When you are done, if you need to save as a JPEG, do a SAVE AS again and select the JPEG file type. That way you are keeping the Photoshop file as well, which means you can easily go back later and do more editing.
Hope this helps.